1. Field of the Invention
This invention is directed to a method of manufacturing a paper substrate used as a base material for a resin-coated laminate. More specifically, the invention is directed to a method of preventing the formation of wavy deformations at the edges of a laminate of a paper sheet having synthetic resin films coated on the both surfaces.
2. Description of the Prior Art
To improve the strength of papers, to render papers waterproof, and to improve the rigidity of synthetic resin films, a so-called laminate paper having the piled structure of a paper sheet and a synthetic resin film has been provided. In particular, to improve the water proof characteristics of papers, a laminate paper having synthetic resin films coated on the both surfaces of a paper has been used.
As the synthetic resins used for the purpose, polyolefins, polyesters, polyamides, cellulose esters, and the like are known. In applying polyolefins to the surface of a paper sheet or web, the molten polyolefin is cast on a travelling paper sheet to provide a coated layer thereon. This method is generally called an extrusion coating method. In the case of coating a polyester or a cellulose ester, a so-called lamination method is usually employed in which the resin films are applied to the both sides of the paper sheet using an adhesive.
In either the laminate product or the unfinished product during the conversion thereof, both sides of the coated laminate sheet tends to be accompanied by the formation of wavy deformations at or near the edge portions thereof. That is to say, when a web of a laminate papers coated on both sides is allowed to stand in a rolled condition, wavy deformations occur at or near both edge portions of the rolled laminate paper and also when a number of squares of a laminate paper coated on both sides is allowed to stand in a piled state, a wavy deformation also occurs at or near the four edge portions of the piled laminates.
If such a wavy deformation occurs in laminate papers, the appearance of the product decreases, which reduces the commercial value of the product, and further the deformation makes it quite difficult to apply a secondary working such as a printing and a coating. Accordingly, the formation of the wavy deformations is one of the most undesirable disadvantages in laminate papers coated on both sides.
The inventors, as the result of much research for preventing the formation of the wavy deformation in laminate papers coated on both sides, have succeeded in obtaining the process of the present invention. The inventors have discovered that the occurrence of the wavy deformations at the edge portions of laminated papers is influenced largely by the water content of the paper substrate at the time of the coating of the synthetic resin, i.e., the weight of water or moisture contained in a unit weight of the paper substrate and further when the water content is greater than about 6% by weight, a laminate paper prepared from the paper substrate and coated on both sides has less or no formation of the wavy deformations at the edge portions. In this case, however, it has also been found that the water content of the paper substrate has an upper limit, that is to say, if the water content is over about 10% by weight, the rigidity of a laminate paper coated on both sides is reduced or the adhesive properties between the resin coatings and the substrate paper decreases. Thus, it has been discovered that in order to effectively prevent the formation of the wavy deformations at the edge portions of a laminate paper coated on both sides without degrading the other properties, the water content of the paper substrate should be from about 6% by weight to about 10% by weight. In other words, when a paper having a water content of from about 6 to about 10% by weight is produced when the paper is manufactured and a laminate paper coated on both sides is prepared using such a paper as the substrate, an improved laminate product unaccompanied by the occurrence of wavy deformations at the edge portions thereof as well as other difficulties in regard to the properties as described above is obtained.
However, the final water content of the papers produced in the conventional paper manufacturing process generally is lower than about 6% by weight or ranges from about 3 to 6% by weight. Incidentally, it sometimes happens that the water content of papers ranges from about 6 to 10% by weight in the transition stage immediately after the start of the manufacturing of the papers but it is quite difficult to produce papers smoothly for a long period of time while maintaining constantly such a high water content in the papers.
As a method of adjusting the water content of papers, in particular, increasing the water content of papers, a method of humidifying the papers by controlling the extent of drying in a dryer or by passing the papers through a wetting zone after drying is known. However, in controlling the extent of drying in the dryer, it is necessary to increase excessively the water content of the papers as a matter of course in the first half of the paper manufacturing process to increase the water content of the final paper product and consequently the papers tend to be cut and further localized undried portions form when the papers are subjected to a smoothing calender or a machine calender. Also, in passing the papers through a wetting zone, large humidifying equipment is required and the cost increases. Thus, these known methods are undesirable commercially.
The process of this invention has been attained as the result of the inventors' research on increasing the water content of the final paper products yet unaccompanied by the aforedescribed difficulties.